All B vitamins help the animal to convert carbohydrates into glucose (sugar), which is "burned" to produce energy.
Thiaminase will reduce this process.

If an animal eats raw animal flesh containing Thiaminase over a period of time of several months then thiaminase will break down the vitamin B1, an essential element of life and render it inactive. Therefore over time the health of your animal will suffer.

But thankfully thiaminase does not stay in the body long however and simply feeding the animal different foods which contain vitamin B1 is all that is required. Alternatively cook the raw flesh first as thiaminase decomposes if heated.